We have studied the influence of nitrogen addition in CH4/H2 gas mixtures on the nucleation, morphology and growth rate of polycrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD). The p resence of nitrogen has little effect on diamond nucleation density, but is bene ficial to enhance the growth rate and to stabilize (100) facets of diamond films . Optical emission spectroscopy was applied to detect the chemical radicals in t he system. It is shown that the additional nitrogen makes the chemical radicals and surface activity of diamond films change tremendously. We believed that nitr ogenous species abstract the atomic hydrogen on the diamond surface and speed up the desorption rate of H, so as to promote the diamond films deposition. Furthe rmore, the selective absorption of nitrogenous species causes chemical roughenin g of the (100) facets, which makes the growth rate of (100) facet higher than th at of other facets and leads to the presence of (100) textured diamond films.